Photoconductive materials have been intensively studied and put into practical use as eletrophotographic photoreceptors, various sensors, and image pickup tubes. Known inorganic photoconductive materials include amorphous selenium, amorphous silicon, cadmium chloride, zinc oxide, and selenium-arsenic alloys. Known organic photoconductive materials include low-molecular materials such as carbazole, anthracene, pyrazolines, oxadiazoles, and hydrazones, and pigments or dyes such as phthalocyanine pigments, azo pigments, cyanine dyes, polycyclic quinone pigments, perylene pigments, and indigo dyes.
Electrophotographic photoreceptors so for widespread comprise a photosensitive layer consisting mainly of an inorganic photoconductive material, e.g., selenium, cadmium, and zinc oxide. These conventional inorganic photoconductive materials are not always satisfactory in sensitivity, heat resistance or printing durability.
Electrophotographic photoreceptors having a photosensitive layer consisting mainly of an organic photoconductive compound, on the other hand, have many advantages, such as relative facility in production, cheapness, non-polluting properties, and the ease on handling. In recent years, considerable attention has been devoted particularly to development of high performance organic photoreceptors in which a carrier generating function and a carrier transport function are respectively performed by different substances as described, e.g., in JP-A-60-67949 (the term "JP-A" as used herein means as "unexamined published Japanese patent application").
On the other hand, gas lasers, e.g., an Ar laser and an He-Ne laser, or semi-conductor lasers are regarded promising as a light source for the electrophotographic photoreceptor of copying machines possessing image processing functions, such as intelligent copying machines, and output printers of computers. Semi-conductor lasers, inter alia, have drawn attention because of the feasibility of reduction in size, weight, and cost. From the fact that the semi-conductor lasers are of lower output as compared with gas lasers and have oscillation wavelengths in the longer wavelength region of more than about 780 nm, some photoconductive compounds having sensitivity in the longer wavelength region have been proposed as disclosed in JP-A-60-19144 and JP-A-60-111248. However, the conventional organic photoconductive compounds, some of them having been turn into practical use, are not necessarily satisfactory in terms of sensitivity, residual potential, and stability on repeated use.
Further, because of the lower output of the semi-conductor lasers as compared with gas lasers and the longer oscillation wavelength (about 780 nm or more) of the semi-conductor lasers as stated above, spectral sensitivity exhibited by the conventional photoreceptors is in the shorter wavelength side. It has been thus demanded to develop a novel compound having high sensitivity in the longer wavelength region corresponding to the oscillation wavelength of the semiconductor lasers.